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Chemical Bonds IV: Hydrogen

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  • 0:21 Intramolecular
  • 1:11 Hydrogen Bonds
  • 6:30 Summary
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Lesson Transcript
Instructor: Robert Egan
Have you ever been curious about why table salt dissolves in water? Well, watch this video and amaze your friends with your knowledge of hydrogen bonds and intermolecular interactions.

Intramolecular Bonds

By this point, we've talked about the different kinds of bonding that can occur between atoms. We've talked about non-polar covalent bonds, polar covalent bonds and ionic bonds. All of these bonds are intramolecular bonds. These are bonds that occur between atoms within the same molecule. We can remember the prefix 'intra' by using a familiar word. I usually use intramural sports as an easy way to remember it.

Diagram of a water molecule
Water Molecule Diagram

We can contrast that with something that is intermolecular, so 'intra' means within, and 'inter' means between. One way of remembering 'inter' is think of an intercontinental ballistic missile, so I think ICBMs, and that's how I remember 'inter.'

So, if we talk about intermolecular bonds, one of the major intermolecular bonds that plays a significant role in biology is hydrogen bonding.

What is Hydrogen Bonding?

Well, we talked about positive charges that can be produced within a molecule. If I have a partial charge within the molecule, that means that electrons are moving toward one of the atoms, which means that I have a partial positive charge given to one atom and a partial negative charge to the other.

Let's consider a partially charged molecule that you're probably pretty familiar with: water. So, water has the molecular formula H2O, which means that there are two hydrogen atoms and one oxygen atom in water. Because it's a polar covalent molecule, let's draw a negative charge here and positive charges here.

The hydrogen atoms are going to spend more time being positive than the oxygen atom, which has a larger electronegativity, so that's drawing the electrons a little bit more towards the oxygen atom than towards the hydrogen atom. How is that going to interact with another molecule?

Sharing electrons to create an ionic bond
Sharing Electrons

Let's talk about another common molecule: salt. The molecular formula for table salt is NaCl, or sodium chloride. What if I tell you that chlorine is the more electronegative atom than sodium? In fact, it's a lot more electronegative than sodium. Pause for a moment and tell me what kind of bond sodium is going to form with chloride.

Did you come up with an ionic bond? Because the electronegative difference between the two atoms is very great, sodium wants to get rid of an electron, and chlorine wants to add an electron.

I'm going to end up with Na+ and Cl-. How do you think this is going to interact with our water molecules?

Attraction of Molecules

We've learned before that positive and negative attract, so which of these two ions do you think is going to be attracted to this partially charged hydrogen? It's probably going to be this negatively charged chlorine. By the same token, this partially charged oxygen atom will be attracted to this positively charged sodium ion.

Diagram of water and sodium chloride molecules
Water and Sodium Chloride Molecules

Recall that we talked about table salt coming in crystals, these nice cubic structures. If you think about it, when you take salt and sprinkle it on your food, it comes out as a nice crystalline structure. But if you were to put this salt into a cup of water, it disappears - you don't see those solids anymore and it vanishes into the water. Once they end up being broken apart by the water molecules, you end up with the salt being dissolved and end up with salt water instead. This is one of the important aspects of hydrogen bonding, the interaction of water molecules with ionic compounds.

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